Campbell Scientific 109 109-L Temperature Probe - Page 17

Model 109 Temperature Probe 8.2 Measurement and Output Linearization Campbell Scientific dataloggers measure the 109 probe thermistor and convert the result to temperature. With reference to the previous FIGURE 8-1, 109 thermistor probe schematic, a precise excitation voltage is applied at the Vx line and the voltage drop across the 24.9 kΩ resistor is measured at the Vs line. The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to thermistor resistance (Rs) and the 24.9 kΩ fixed resistor as described in the following equations: Vs/Vx = 24900 Ω / (Rs + 24900 Ω) Solving for Rs: Rs + 24900 Ω = 24900 Ω • (Vx/Vs) Rs = 24900 Ω • ((Vx/Vs) - 1) TABLE 8-1, 109 Measurement Details, and TABLE 8-2, 109 Temperature Calculation, describe how measurement results Vs/Vx and Rs are converted to temperature by Campbell Scientific dataloggers. TABLE 8-1. 109-Measurement Details Datalogger Model CR200(X) Series CR800 CR1000 CR3000 CR5000 CR500 CR510 CR10 CR10X 21X CR7(X) CR23X Measurement Instruction CRBasic Therm109() Edlog AC Half Bridge (P5) Edlog AC Half Bridge (P5) Excite mV 2500 5000 Voltage Input Range 2500 mV 5000 mV Result Vs/Vx Vs/Vx Scaling ln(Rs) by 1E-3 ln(Rs) by 1E-3 Equation Applied to Scaled Result Steinhart-Hart (automatically applied) Steinhart-Hart (use Polynomial (P55)) Steinhart-Hart (use Polynomial (P55)) 11